Ⅰ.
combine/join forces
► to work with someone in order to achieve something you both want: »
The two companies, one Dutch the other French, have just joined forces to exploit the European market for petfood.
Main Entry: ↑force
Ⅱ.
join forces
► to act with somebody else in order to do something: »
The two companies have joined forces to make the Web applications available to a wider public.
Main Entry: ↑join
Financial and business terms. 2012.